Objective Lens and Optical Pickup Apparatus

ABSTRACT

An objective lens is provided for an optical pickup apparatus at least reproducing information for a first optical disc, and recording and/or reproducing information for second and a third optical discs. The objective lens includes an optical surface including a common area including a diffractive structure having a cross section in a serrated shape and divided into a plurality of ring-shaped zones. Refractive index differences between media arranged at both side of the optical surface for a light flux with one and the other wavelength satisfy a predetermined condition, and the objective lens satisfies a predetermined conditions defined by using an approximate coefficient which makes a Pearson&#39;s correlation coefficient R satisfy 0.99998≦R when a depth parallel to the optical axis of step differences between the ring-shaped zones at a vertical distance from the optical axis is approximated by a least squares method.

TECHNICAL FIELD

The present invention relates to an objective lens and a pickupapparatus.

BACKGROUND ART

In recent years, there has been advanced a trend towards a shorterwavelength of a laser light source used as a light source forreproducing of information recorded on an optical disc and for recordinginformation on an optical disc, and a laser light source with awavelength of 405 nm such as, for example, a violet semiconductor laseror a violet SHG laser that conducts wavelength conversion of an infraredsemiconductor laser by utilizing second harmonic generation has been putto practical use.

When these violet laser light sources are used, information in volume of15-20 GB can be recorded on an optical disc having a diameter of 12 cm,in the case of using an objective lens having a numerical aperture (NA)identical to that of a digital versatile disc (hereinafter referred toas DVD for short), and information in volume of 23-27 GB can be recordedon an optical disc having a diameter of 12 cm, in the case of enhancingNA of the objective lens to 0.85. Hereafter, an optical disc employing aviolet laser light source and a magnetic optical disc are called “highdensity optical disc” generically in the present specification.

Incidentally, as a high density optical disc, there are proposed twostandards at present. One of them is Blu-Ray disc (hereinafter referredto as BD for short) that uses an objective lens with NA 0.85 and has aprotective layer whose thickness is 0.1 mm, and the other is HD DVD(hereinafter referred to as HD for short) that uses an objective lenswith NA 0.65-0.67 and has a protective layer whose thickness is 0.6 mm.When considering possibility of circulation of these two high densityoptical discs in the two standards on the market, compatible opticalpickup apparatuses which can conduct recording and/or reproducing fornot only existing DVD and CD but also for all types of high densityoptical discs are important, and a one-lens system that is compatible byusing a common objective lens for a high density disc, DVD and CD amountthem is most ideal.

With respect to an optical pickup apparatus having compatibility forplural types of optical discs, there have been known technologies tochange a degree of divergence for a light flux entering an objectiveoptical system or to provide a diffractive structure on an opticalsurface of an optical element constituting an optical pickup apparatus,as a method of correcting spherical aberration caused by differences ina light flux wavelength and a protective layer thickness used inrespective optical discs (for example, see Patent Document 1).

However, when achieving compatibility for BD, HD DVD and CD, coma of anobjective lens caused by its tracking is great, and preferable recordingsignals and reproducing signals cannot be obtained accordingly, whentrying to achieve compatibility only by a method to change a degree ofdivergence of a light flux entering an objective optical system(hereinafter referred to as an objective lens), because a difference ofsubstrate between respective optical discs is great. On the other hand,when trying to achieve compatibility by providing a diffractivestructure on an optical element and by utilizing diffracting effects,high amount of light of 90% or more cannot be obtained because of awavelength ratio of violet laser light used for high density disc toinfrared laser light used for CD. Further, since wavelength dependencyof the diffractive structure is determined by the design for achievingcompatibility, if a wavelength of each incident light entering anobjective lens is varied or changed, wavefront aberration is affected.Patent Document 1: TOKUKAI No. 2002-298422

The invention described in Patent Document 1 is one to record andreproduce information for respective optical discs by making opticalsystem magnification of the objective lens for each of plural types ofoptical discs to be 0. In this method, however, a change in wavelengthcharacteristic caused by diffracting effects grows greater, andwavefront aberration is worsened, when an oscillation wavelength isvaried among laser lots, although coma is hardly generated by lensshifting in the course of tracking, which is a problem. Further, thereis a problem that an objective lens seems most likely to be designedgiving top priority to the high density disc, and light utilizationefficiency for the lowest density optical disc is insufficient, althoughlight utilization efficiency for CD representing an optical disc oflowest density is improved. Therefore, it is difficult to realize higherspeed of recording and reproducing information for the lowest densityoptical disc which is more specifically CD, which is a problem.

Further, when the technology disclosed in the Patent Document 1 isapplied as it is, there are problems that an increase of manufacturingcost for optical pickup apparatuses and worsening efficiency ofmanufacturing operations are caused.

DISCLOSURE OF INVENTION

In view of the problems mentioned above, an objective of the inventionis to provide an objective lens wherein the light utilization efficiencyfor the lowest density optical disc is improved to be easily applied toan optical pickup apparatus capable of recording or reproducing at highspeed, then, different optical discs of three types or more including ahigh density disc can be handled compatibly, and excellent wavelengthcharacteristics and tracking characteristics are provided, and toprovide an optical pickup apparatus employing the objective lens.

To solve the above problems, a structure descried in Item 1 is anobjective lens for an optical pickup apparatus at least reproducinginformation using a light flux with a wavelength λ1 emitted from a firstlight source for a first optical disc having a protective substrate witha thickness t1, recording and/or reproducing information using a lightflux with a wavelength λ2 (1.5×λ1≦λ2≦1.7×λ1) emitted from a second lightsource for a second optical disc having a protective substrate with athickness t2, and recording and/or reproducing information using a lightflux with a wavelength λ3 (1.8×λ1≦λ3≦2.2×λ1) emitted from a third lightsource for a third optical disc having a protective substrate with athickness t3 (1.9×t1≦t3≦2.1×t1). The objective lens includes: at leastone optical surface including a common area for reproducing informationfor the first disc and for recording and/or reproducing information forthe second and third discs; and a diffractive structure arranged on thecommon area, having a cross section in a serrated shape and divided intoa plurality of ring-shaped zones whose centers are on an optical axis.When the optical surface including the diffractive structure forms aborder, a refractive index difference n_(d1) for a light flux with thewavelength λ1 between a medium A arranged at a light source side of theoptical surface and a medium B arranged at an optical disc side of theoptical surface, and refractive index difference n_(d2) for a light fluxwith the wavelength λ2 between the medium A and the medium B satisfies57≦|(n _(d2) −n _(d1))/(λ2−λ1)|≦90.

When the plurality of ring-shaped zones have step differences betweenevery pair of the ring-shaped zones each having a depth di [mm] parallelto an optical axis and a vertical distance hi [mm] from the opticalaxis, the objective lens satisfies any one of:3.38≦α≦3.45,5.37≦α≦5.46,7.25≦α≦7.39,9.38≦α≦9.45, and11.41≦α≦11.43,

where α is defined by a==C₀×|n_(d1)/λ1| using an approximate coefficientC₀ which makes a Pearson's correlation coefficient R satisfy 0.99998≦Rwhen for the step differences whose number m satisfies m>7 arrangedbetween every pair of the ring-shaped zones, a depth d parallel to theoptical axis at a vertical distance hi from the optical axis isapproximated by a least squares method using a following expression (1)to obtain C_(2k) (k is an integer in a range of 0 to 5) and isdetermined by using C_(2k), and a whole of the step differences exceptstep differences providing maximum and minimum differences between acalculated value of the depth d parallel to the optical axis at thevertical distance hi from the optical axis and di, are approximatedagain using the expression (1). Wherein for the step differences whosenumber m satisfies m≦7 arranged between every pair of the ring-shapedzones, when the depth d parallel to the optical axis at a verticaldistance hi from the optical axis is determined, C_(2k) (k is an integerin a range of 0 to m−3) is obtained by approximating by the leastsquares method using a following expression (2) instead of theexpression (1), and wherein when a number of the calculated values ofthe depth d parallel to the optical axis at a vertical distance hi fromthe optical axis is 3 or less, a whole of the step differences includingthe step differences providing the maximum and minimum differencesbetween the calculated value of the depth d parallel to the optical axisat a vertical distance hi from the optical axis and di, are approximatedagain. $\begin{matrix}{{di} = {\sum\limits_{k = 0}^{5}{\left( {C_{2k} \cdot h_{i}^{2k}} \right)\left( {{i = 1},2,{3\ldots\quad m}} \right)}}} & (1) \\{{di} = {\sum\limits_{k = 0}^{m - 3}{\left( {C_{2k} \cdot h_{i}^{2k}} \right)\left( {{i = 1},2,{3\ldots\quad m}} \right)}}} & (2)\end{matrix}$

Where, C_(2k) is a constant, k is an integer, i is a natural number, andm is a number of the step differences between every pair of thering-shaped zones.

In a structure described in Item 2, according to the objective lens ofItem 1, α has an integer part of 3.

A diffractive structure formed on an optical surface of the objectivelens is a structure to correct spherical aberration caused by athickness difference in the first, second and third optical discs,and/or wavefront aberration caused by changes in refractive index of theobjective lens resulted from changes in ambient temperature and inoscillation wavelength.

By setting α to either one of the aforesaid ranges as in the structuredescribed in Item 1, in the objective lens having characteristics of(n_(d2)−n_(d1))/(λ2−λ1), the diffraction efficiency arrives at 50% ormore for the first optical disc and at 70% or more for the third opticaldisc, and sufficient amount of light can be secured as a use forexclusive reproduction of information for the high density optical discas the first optical disc, and sufficient amount of light of the secondlight flux and the third light flux for reproducing and/or recording ofinformation for DVD and CD representing respectively the second opticaldisc and the third optical disc.

Further, the diffractive structure includes a plurality of ring-shapedzones 100 in terms of a shape as shown typically in FIGS. 1(a) and 1(b),and a sectional form including an optical axis is serrated.Incidentally, though each of FIGS. 1(a) and 1(b) shows typically anexample where a diffractive structure is formed on a plane, thediffractive structure may also be formed on a spherical surface or on anaspheric surface. In the mean time, in the present specification, it isassumed that symbol “DOE” represents a diffractive structure includingplural ring-shaped zones shown in FIGS. 1(a) and 1(b). With respect tothis diffractive shape, a width of each ring-shaped zone in the verticaldirection from the optical axis is determined to correct the aforesaidwavefront aberration, and a depth of each ring-shaped zone that is inparallel to the optical axis is determined for the diffractionefficiency for each of light fluxes having respectively wavelengths λ2to λ3.

In the present specification, when a thickness of a coating layer in thedirection parallel with an optical axis is smaller than a depth of eachring-shaped zone being in parallel with the optical axis, a coatinglayer such as an antireflection coating given to the lens is notincluded in medium A and medium B. However, when a thickness of acoating layer that is in parallel with an optical axis is greater than adepth of each ring-shaped zone that is in parallel with an optical axis,a coating layer is also included in medium A or medium B. The reason forthis is that a phase given to incident light by a depth of diffractivering-shaped zone is dependent on the refractive index of the coatinglayer.

When the diffractive structure is set so that an integer part of α maybe an odd number, as in the structure described in Items 1 and 2, itprovides the light flux with wavelength λ1 an optical path differencethat is substantially a multiple of an odd number of the wavelength, andit generates mainly N^(th) order diffracted light and (N−1)^(th) orderdiffracted light which are substantially the same in terms ofdiffraction efficiency from the light flux with wavelength λ3(1.8×λ1≦λ3≦2.2×λ1) entering the aforesaid diffractive structure. Here,when an example that N^(th) order diffracted light having smallerspherical aberration at the magnification identical to optical systemmagnification m1 of the objective lens for the light flux withwavelength λ1 is used for reproducing and/or recording for the thirdoptical disc is compared with an example that distance d of a stepdifference is established so as to provide the light flux withwavelength λ1 an optical path difference that is substantially amultiple of an even number of the wavelength, and the diffracted lighthaving the maximum diffraction efficiency among light fluxes withwavelength λ3 generated in the course of passing through thisdiffractive structure is used for reproducing and/or recording, in thesetwo diffracted lights, the optical system magnification for correctingspherical aberration is closer to 0 and aberration caused in the courseof tracking can be made smaller in the former than in the latter.

Meanwhile, from the viewpoint of prevention of a decline of thediffraction efficiency in the course of wavelength fluctuations, thelower order of diffraction for the diffracted light is preferable, andif an integer part of α is made to be 3 as in Item 2, the optical systemmagnification of the objective lens for the first and second opticaldiscs becomes zero substantially, and performance for coma in the courseof tracking, temperature characteristics and wavelength characteristicsturn out to be excellent.

Further, a structure described in Item 16 is an objective lens for anoptical pickup apparatus, at least reproducing information using a lightflux with a wavelength λ1 emitted from a first light source for a firstoptical disc having a protective substrate with a thickness t1,recording and/or reproducing information using a light flux with awavelength λ2 (1.5×λ1≦λ2≦1.7×λ1) emitted from a second light source fora second optical disc having a protective substrate with a thickness t2,and recording and/or reproducing information using a light flux with awavelength λ3 (1.8×λ1≦λ3≦2.2×λ1) emitted from a third light source for athird optical disc having a protective substrate with a thickness t3(1.9×t1≦t3≦2.1×t1). The objective lens includes: at least one opticalsurface having a common area for reproducing information for the firstdisc and for recording and/or reproducing information for the second andthird discs; and a diffractive structure arranged on the common areawhose diffraction efficient of the light flux with a wavelength λ1 forthe first optical disc is 50% or more, and whose diffraction efficientof a light flux with the wavelength λ2 for the second optical disc is70% or more.

In the present specification, it is assumed that the high densityoptical disc includes an optical disc having, on its informationrecording surface, a protective layer with a thickness of about severalnanometers—several tens of nanometers and an optical disc having aprotective layer or a protective film whose thickness is 0 (zero), inaddition to the aforesaid BD and HD.

In the present specification, DVD is a generic name of optical discs ofDVD series such as DVD-ROM, DVD-Video, DVD-Audio, DVD-RAM, DVD-R,DVD-RW, DVD+R and DVD+RW, and CD is a generic name of optical discs ofCD series such as CD-ROM, CD-Audio, CD-Video, CD-R and CD-RW.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows figures (a) and (b) each representing a diffractivestructure.

FIG. 2 is a top plan view of primary portions showing the structure ofan optical pickup apparatus.

FIG. 3 is a top plan view of primary portions showing the structure ofan objective lens.

FIG. 4 is an enlarged top plan view of primary portions showing thestructure of an objective lens.

FIG. 5 is an enlarged top plan view of primary portions showing thestructure of an objective lens.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the invention will be explained as follows.

In a structure described in Item 3, according to the objective lensdescribed in any one of Items 1 and 2, the objective lens is formed oftwo lenses.

A structure described in Item 4, according to the objective lensdescribed in Item 3, is characterized in that the diffractive structureis on a lens arranged at a light source side between the two lenses.

In a structure described in Item 5, according to the objective lensdescribed in any one of Items 1 to 4, the objective lens satisfies60≦|(n_(d2)−n_(d1))/(λ2−λ1)|≦80.

A structure described in Item 6, according to the objective lensdescribed in any one of Items 1 to 5, is characterized in that thediffractive structure is defined by using an optical path differencefunction φ(h) whereφ(h)=B ₂ ×h ² +B ₄ ×h ⁴ + . . . +B _(2i) ×h ^(2i), andB₄<0.

Where, B_(2i) is a coefficient of the optical path difference functionand i is a natural number.

When coefficient B₄<0 is made to hold as in the structure described inItem 6, the diffractive structure results in having positive diffractivefunctions, and spherical aberration caused by the diffractive functionsin the case of temperature changes can be canceled with the diffractivefunctions by changing an oscillation wavelength of a laser (lightsource).

Since the diffracted light with wavelength λ1 generated by passingthrough the diffractive structure has a diffraction efficient with theopposite sign to spherical aberration caused in the course of changes inwavelength depending on lens material, it is possible to correctspherical aberration characteristics in the case of wavelength changeand temperature change. Since an amount of spherical aberration in thecase of wavelength change and temperature change is proportional to thefourth power of NA, it is more effective if this technology is used forBD having higher NA.

Incidentally, to keep balance between wavelength characteristics andtemperature characteristics, it is preferable to make coefficient B₄ tobe within a range of −1.0×10⁻³<B₄<−1.0×10⁻⁴, and it is more preferableto make coefficient B₄ to be within a range of −7.0×10⁻⁴<B₄<−4.5×10⁻⁴,when a material of the objective lens is an ordinary optical resin suchas, for example, “ZEONEX340R” (product name) of ZEON Corporation or“APEL” (product name) of Mitsui Chemicals, Inc.

In a structure described in Item 7, according to the objective lensdescribed in any one of Items 1 to 6, the diffractive structure has anegative diffraction power.

By providing the negative diffraction power as described in Item 7,chromatic aberration in the light flux with the wavelength λ1 or λ2 whenrecording and/or reproducing of information for the first optical discand the second optical disc is conducted, can be corrected well.

In a structure described in Item 8, according to the objective lensdescribed in Item 7, a change amount of a position dfb/dλ where awavefront aberration is minimum along the optical axis in the objectivelens for a light flux with the wavelength λ1 per a wavelength change of1 nm satisfies|dfb/dλ|≦0.1 [μm/nm].

Where, fb is a distance between the objective lens and the first opticaldisc.

A structure descried in Item 9, according to the objective lens of Item7, a change amount of a position dfb/dλ where a wavefront aberration isminimum along the optical axis in the objective lens for a light fluxwith the wavelength λ1 per a wavelength change of 1 nm satisfies|dfb/dλ|≦0.1 [μm/nm].

Where, fb is a distance between the objective lens and the first opticaldisc.

Meanwhile, in the aforesaid structure, it is preferable that at leastone of optical system magnifications m1, m2 and m3 of the aforesaidobjective lens for light fluxes respectively with wavelengths λ1, λ2 andλ3 is not 0 in the objective lens described in any one of Items 1 to 9.

Further, it is more preferable that the aforesaid magnification which isnot 0 is not less than − 1/10 and is less than − 1/100, in the aforesaidobjective lens.

As stated above, by setting so that an optical system magnification onat least one side among optical system magnifications m1, m2 and m3 maynot be 0 (preferably, within a range from − 1/10 or more to less than −1/100), compatibility for the first, second the third optical discs isshared by diffractive functions and magnification changes, whereby,wavelength-dependency of diffractive functions does not become too greatand no problem is caused for operations even in the case wherewavelength only changes like fluctuations of oscillation wavelengthbetween laser lots.

Further, coma caused in the case of tracking of the objective lensresults in optical system magnification at the level which makesreproducing and/or recording to be possible.

In a structure described in Item 10, according to the objective lensdescribed in any one of Items 1 to 9, among optical systemmagnifications m1, m2, and m3 of the objective lens for the wavelengthλ1, λ2 and λ3, m1=0 and at least one of m2 and m3 satisfies a followingexpression.1/100<|m|≦ 1/10.

As descried in Item 10, by setting so that m1 among optical systemmagnifications m1, m2 and m3 is zero, and at least one of optical systemmagnifications m2 and m3 satisfies the relation of 1/100<|m|≦ 1/10,compatibility of the first optical disc, the second optical disc and thethird optical disc is shared by diffractive actions and by magnificationchanges, whereby, wave-dependency of the diffractive actions does notgrow too great, and no problem is caused for operations, even in thecase of changes of only waves such as fluctuations of oscillationwavelengths between laser lots. Further, coma caused in the course oftracking of an objective lens becomes an optical system magnification onthe level capable of reproducing and/or recording. In particular, it ispossible to improve reproducing capability of information for the firstoptical disc that employs a light flux with wavelength λ1 that is asmallest wavelength.

A structure described in Item 11, according to the objective lensdescribed in any one of Items 1 to 10, satisfies t1=t2.

In the structure described in Item 11, only spherical aberration causedby chromatic aberration for each of wavelength λ1 and wavelength λ2 hasonly to be corrected, for attaining compatibility between the firstoptical disc and the second optical disc, whereby, it is possible toreduce both a difference between optical system magnifications m1 and m2and wavelength-dependency of diffractive actions.

In the aforesaid structure, it is preferable that NA1 is equal to NA2,when NA1 represents an image side numerical aperture of the objectivelens for the aforesaid wavelength λ1 and NA2 represents an image sidenumerical aperture of the objective lens for the aforesaid wavelengthλ2, in the objective lens described in any one of Items 1 to 10.

In the present embodiment, a difference of effective diameters ofobjective lenses for conducting reproducing and/or recording for thefirst optical disc and the second optical disc is small, and it is notnecessary to provide an aperture restriction individually accordingly.

In the aforesaid structure, it is preferable that an aperturerestriction element is provided on the optical path for a light fluxwith the wavelength λ3 and between the third light source and theobjective lens, in the objective lens described in any one of Items 1 to10.

The present structure makes it possible to conduct aperture restrictionfor the light flux with wavelength λ3.

In the aforesaid structure, it is preferable that a chromatic aberrationcorrecting element having functions to correct chromatic aberration fora passing light flux is provided on at least one of an optical path fora light flux with wavelength λ1 and an optical path for a light fluxwith wavelength λ2, in the objective lens described in any one of Items1 to 10.

It is further preferable that the chromatic aberration correctingelement is a collimator lens in the aforesaid objective lens.

In the aforesaid structure, even in the case where chromatic aberrationcan be corrected by an objective lens only for the light flux on oneside among two light fluxes respectively with wavelength λ1 andwavelength λ2, chromatic aberration can be corrected by the chromaticaberration element also for the wavelength on the other side. As thischromatic aberration correcting element, there is given, for example, acollimator lens.

In a structure described in Item 12, according to the objective lensdescribed in any one of Items 1 to 11, a focal length f1 of theobjective lens for a light flux with the wavelength λ1 satisfies 0.8mm≦f1≦4.0 mm.

In a structure described in Item 13, according to the objective lensdescribed in any one of Items 1 to 12, the objective lens is formed of aplastic.

In a structure described in Item 14, according to the objective lensdescribed in any one of Items 1 to 20, the objective lens satisfies0.9×t1≦t2≦1.1×t1.

In a structure described in Item 15, according to the objective lensdescribed in any one of Items 1 to 14, one of the medium A and medium Bis an air and the other is a lens material of the lens including thediffractive structure.

In a structure described in Item 17, according to the objective lensdescribed in Item 16, the diffractive structure has a cross section in aserrated shape is divided into a plurality of ring-shaped zones whosecenters are on an optical axis.

In a structure described in Item 18, according to the objective lensdescribed in Item 16 or 17, when the optical surface including thediffractive structure forms a border, a refractive index differencen_(d1) for a light flux with the wavelength λ1 between a medium Aarranged at a light source side of the optical surface and a medium Barranged at an optical disc side of the optical surface, and refractiveindex difference n_(d2) for a light flux with the wavelength λ2 betweenthe medium A and the medium B satisfy57≦|(n _(d2) −n _(d1))/(λ2−λ1)|≦90.

In a structure described in Item 19, according to the objective lensdescribed in Item 17, when the plurality of ring-shaped zones have stepdifferences between every pair of the ring-shaped zones each having adepth di [mm] parallel to an optical axis and a vertical distance hi[mm] from the optical axis, the objective lens satisfies any one of:3.38≦α≦3.45,5.37≦α≦5.46,7.25≦α≦7.39,9.38≦α≦9.45, and11.41≦α≦11.43,

where α is defined byα==C ₀ ×|n _(d1)/λ1|

using an approximate coefficient C₀ which makes a Pearson's correlationcoefficient R satisfy 0.99998≦R when for the step differences whosenumber m satisfies m>7 arranged between every pair of the ring-shapedzones, a depth d parallel to the optical axis at a vertical distance hifrom the optical axis is approximated by a least squares method using afollowing expression (1) to obtain C_(2k) (k is an integer in a range of0 to 5) and is determined by using C_(2k), and a whole of the stepdifferences except step differences providing maximum and minimumdifferences between a calculated value of the depth d parallel to theoptical axis at the vertical distance hi from the optical axis and di,are approximated again using the expression (1). Wherein for the stepdifferences whose number m satisfies m≦7 arranged between every pair ofthe ring-shaped zones, when the depth d parallel to the optical axis ata vertical distance hi from the optical axis is determined, C_(2k) (k isan integer in a range of 0 to m−3) is obtained by approximating by theleast squares method using a following expression (2) instead of theexpression (1), and wherein when a number of the calculated values ofthe depth d parallel to the optical axis at a vertical distance hi fromthe optical axis is 3 or less, a whole of the step differences includingthe step differences providing the maximum and minimum differencesbetween the calculated value of the depth d parallel to the optical axisat a vertical distance hi from the optical axis and di, are approximatedagain. $\begin{matrix}{{di} = {\sum\limits_{k = 0}^{5}{\left( {C_{2k} \cdot h_{i}^{2k}} \right)\left( {{i = 1},2,{3\ldots\quad m}} \right)}}} & (1) \\{{di} = {\sum\limits_{k = 0}^{m - 3}{\left( {C_{2k} \cdot h_{i}^{2k}} \right)\left( {{i = 1},2,{3\ldots\quad m}} \right)}}} & (2)\end{matrix}$

Where, C_(2k) is a constant, k is an integer, i is a natural number, andm is a number of the step differences between every pair of thering-shaped zones.

In a structure described in Item 20, according to the objective lensdescribed in Item 19, α has an integer part of 3.

In a structure described in Item 21, according to the objective lensdescribed in any one of Items 16 to 19, the objective lens is formed oftwo lenses.

In a structure described in Item 22, according to the objective lensdescribed in Item 21, the diffractive structure is on a lens arranged ata light source side between the two lenses.

In a structure described in Item 23, according to the objective lensdescribed in any one of Items 18 to 22, the objective lens satisfies60≦|(n_(d2)−n_(d1))/(λ2−λ1)|≦80.

A structure described in Item 24, according to the objective lensdescribed in any one of Items 1 to 15, includes: a first light sourcefor reproducing information using a light flux with a wavelength λ1emitted from the first light source for a first optical disc having aprotective substrate with a thickness t1; a second light source forrecording and/or reproducing information using a light flux with awavelength λ2 (1.5×λ1≦λ2≦1.7×λ1) emitted from the second light sourcefor a second optical disc having a protective substrate with a thicknesst2; a third light source for recording and/or reproducing informationusing a light flux with a wavelength λ3 (1.8×λ1≦λ3≦2.2×λ1) emitted fromthe third light source for a third optical disc having a protectivesubstrate with a thickness t3 (1.9×t1≦t3≦2.1×t1); and the objective lensdescribed in any one of Items 16 to 23.

A structure described in Item 25, according to the objective lensdescribed in any one of Items 16 to 23, includes: a first light sourcefor reproducing information using a light flux with a wavelength λ1emitted from the first light source for a first optical disc having aprotective substrate with a thickness t1; a second light source forrecording and/or reproducing information using a light flux with awavelength λ2 (1.5×λ1≦λ2≦1.7×λ1) emitted from the second light sourcefor a second optical disc having a protective substrate with a thicknesst2; a third light source for recording and/or reproducing informationusing a light flux with a wavelength λ3 (1.8×λ1≦λ3≦2.2×λ1) emitted fromthe third light source for a third optical disc having a protectivesubstrate with a thickness t3 (1.9×t1≦t3≦2.1×t1); and the objective lensdescribed in any one of Items 16 to 23.

The best mode to carrying out the invention will be explained in detailas follows, referring to drawings.

FIG. 2 is a diagram showing schematically the structure of opticalpickup apparatus PU which can conduct only reproducing of informationproperly for HD (first optical disc) and can conduct reproducing and/orrecording of information properly for DVD (second optical disc) and CD(third optical disc).

Optical specifications of HD include wavelength λ1=407 nm, thickness t1of protective layer (protective substrate) PL1=0.6 mm and numericalaperture NA1=0.65, optical specifications of DVD include wavelengthλ2=655 nm, thickness t2 of protective layer PL2=0.6 mm and numericalaperture NA2=0.65, and optical specifications of CD include wavelengthλ3=785 nm, thickness t3 of protective layer PL3=1.2 mm and numericalaperture NA3=0.51.

However, a combination of the wavelength, a thickness of a protectivelayer and a numerical aperture is not limited to the foregoing. Further,as the first optical disc, BD whose thickness t1 of protective layer PL1is about 0.1 mm may also be used.

For m1, m2 and m3 each representing an optical system magnification ofthe objective lens, there are respectively relations of m1=0, −1/10≦m2≦− 1/100 and − 1/10≦m3≦− 1/100. Namely, objective lens OBJ in thepresent embodiment has the structure wherein the first light flux entersas collimated light, while, each of the second and third light fluxesenters as gentle divergent light. In the invention, however, it is notalways needed that optical system magnifications m1, m2 and m3 of theobjective lens are in the aforesaid range.

Optical pickup apparatus PU includes violet semiconductor laser LD1(first light source) that emits a laser light flux (first light flux)with wavelength of 407 nm when reproducing information for high densityoptical disc HD; photodetector PD1 for the first light flux; lightsource unit LU wherein red semiconductor laser LD2 (second light source)that emits a laser light flux (second light flux) with wavelength of 655nm when conducting reproducing and/or recording of information for DVDand infrared semiconductor laser LD3 (third light source) that emits alaser light flux (third light flux) with wavelength of 785 nm whenconducting reproducing and/or recording of information for CD are unitedsolidly; photodetector PD2 that is common for the second and third lightfluxes; first collimator lens COL1 through which the first light fluxalone passes; second collimator lens COL2 through which the second andthird light fluxes pass; objective lens OBJ having a function toconverge laser light fluxes on information recording surfaces RL1, RL2and RL3 wherein a diffractive structure is formed on its optical surfaceas a phase structure, and its both sides are aspheric surfaces; firstbeam splitter BS1; second beam splitter BS2; third beam splitter BS3;diaphragm STO; and lens sensors SEN1 and SEN2.

In the optical pickup apparatus PU, when reproducing information for HD,the violet semiconductor laser LD1 is first caused to emit light as itsbeam path is shown with solid lines in FIG. 2. A divergent light fluxemitted from the violet semiconductor laser LD1 passes through the firstbeam splitter BS1 to arrive at the first collimator lens COL1.

Then, the first light flux is converted to collimated light when it istransmitted through the first collimator lens COL1, and passes throughthe second beam splitter BS2 and quarter wavelength plate RE to arriveat objective optical element OBJ, thus, it becomes a spot that is formedby objective lens OBJ on information recording surface RL1 through firstprotective layer PL1. Focusing and tracking for the objective lens OBJare carried out by biaxial actuator AC1 arranged around the objectivelens OBJ.

A reflected light flux modulated by information pits on informationrecording surface RL1 passes again through objective lens OBJ, quarterwavelength plate RE, second beam splitter BS2 and first collimator lensCOL1, then, is branched on the first beam splitter BS1, and is givenastigmatism by sensor lens SEN1 to be converged on a light-receivingsurface of photodetector PD1. Thus, it is possible to read informationrecorded on HD by using output signals of photodetector PD1.

Further, when conducting reproducing and/or recording of information forDVD, red semiconductor laser LD2 is first caused to emit light as itsbeam path is shown with dotted lines in FIG. 2. A divergent light fluxemitted from the red semiconductor laser LD2 passes through the thirdbeam splitter BS3 to arrive at the second collimator lens COL2.

Then, the second light flux is converted to gentle divergent light whenit is transmitted through the second collimator lens COL2, and isreflected on the second beam splitter BS2 to arrive at objective lensOBJ after passing through quarter wavelength plate RE, and it becomes aspot that is formed by objective lens OBJ on information recordingsurface RL2 through second protective layer PL2. Focusing and trackingfor the objective lens OBJ are carried out by biaxial actuator AC1arranged around the objective lens OBJ.

Or, the second light flux may also be converted into gentle convergentlight when it is transmitted through the second collimator lens COL2, sothat the convergent light is reflected on the second beam splitter BS2to enter objective lens OBJ after passing through quarter wavelengthplate RE.

A reflected light flux modulated by information pits on informationrecording surface RL2 passes again through objective lens OBJ andquarter wavelength plate RE, and after it is reflected on the secondbeam splitter BS2, it passes through collimator lens COL2 and isbranched by third beam splitter BS3 to be converged on a light-receivingsurface of photodetector PD2. Thus, it is possible to read informationrecorded on DVD by using output signals of photodetector PD2.

When conducting reproducing and/or recording of information for CD, theinfrared semiconductor laser LD3 is first caused to emit light as itsbeam path is shown with two-dot chain lines in FIG. 2. A divergent lightflux emitted from the infrared semiconductor laser LD3 passes throughthe third beam splitter BS3 to arrive at the second collimator lensCOL2.

Then, the third light flux is converted to gentle divergent light whenit is transmitted through the second collimator lens COL2, and isreflected on the second beam splitter BS2 to arrive at objective lensOBJ after passing through quarter wavelength plate RE, and it becomes aspot that is formed by objective lens OBJ on information recordingsurface RL3 through third protective layer PL3. Focusing and trackingfor the objective lens OBJ are carried out by biaxial actuator Ac1arranged around the objective lens OBJ.

A reflected light flux modulated by information pits on informationrecording surface RL3 passes again through objective lens OBJ andquarter wavelength plate RE, and after it is reflected on the secondbeam splitter BS2, it passes through collimator lens COL2 and isdiverged by third beam splitter BS3 to be converged on a light-receivingsurface of photodetector PD2. Thus, it is possible to read informationrecorded on CD by using output signals of photodetector PD2.

Next, a structure of objective lens OBJ will be explained.

As shown in FIG. 3, objective lens OBJ is a plastic single lens of asingle-group single-element type.

Diffractive structure DOE is formed on common area RC used forreproducing of the first disc and for reproducing and/or recording ofthe second and third optical discs, on optical surface S1 of objectivelens OBJ, and light-emerging surface S2 is a refractive interface.

The diffractive structure DOE includes plural ring-shaped zones R (R1 toRn) in a shape of concentric circles each having their centers onoptical axis L, and a cross-section of the diffractive structureincluding optical axis L is in a serrated shape.

The optical surface on which the diffractive structure is formed by air(medium A) closer to the light source and medium (medium B) of theobjective lens closer to the optical disc, both existing respectively onboth sides of the optical surface as a border. Since the refractiveindex of air is 1 for light having any wavelength, n_(d2)−n_(d1)=n₁−n₂holds when n₁ represents the refractive index of the medium of theobjective lens for light with wavelength λ1, and n₂ represents therefractive index of the medium of the objective lens for light withwavelength λ2. Therefore, (n_(d2)−n_(d1))/(λ2−λ1)=(n₁−n₂)/(λ2−λ1) holds,and this indicates wavelength-dependency of the refractive index ofmedium B, and it satisfies 5.7×10⁵≦|(n_(d2)−n_(d1))/(λ2−λ1)|≦9.0×10⁵.

Further, under the assumption that a step difference between a pair ofring-shaped zones in plural ring-shaped zones has a depth di [mm]parallel to the optical axis and a vertical distance hi [mm] from theoptical axis, when the number m of the step differences between thering-shaped zones is greater than 7, depth d parallel to the opticalaxis in the vertical distance hi from the optical axis is determined byusing C_(2k) (k is an integer from 0 to 5) obtained by approximatingwith a least squares method by the use of the following expression (1),and a whole of the step differences except step differences providingmaximum and minimum differences between a calculated value of the depthd parallel to the optical axis at the vertical distance hi from theoptical axis and di, are approximated again. When the number m of thestep differences between the ring-shaped zones is equal to or smallerthan 7, in the determination of the depth d parallel to the optical axisat a vertical distance hi from the optical axis, C_(2k) (k is an integerfrom 0 to m−3) is obtained by approximating with the least squaresmethod by using the following expression (2) instead of the expression(1). When the number of the calculated values of the depth d parallel tothe optical axis at a vertical distance hi from the optical axis is 3 orless, a whole of the step differences are approximated again withoutremoving the step differences providing the maximum and minimumdifferences between the calculated value of the depth d parallel to theoptical axis at a vertical distance hi from the optical axis and di. Theoptical surface on which the aforesaid diffractive structure is formed,satisfies any one of:3.38≦α≦3.45,5.37≦α≦5.46,7.25≦α≦7.39,9.38≦α≦9.45, and11.41≦α≦11.43,

where α is defined with α==C₀×|n_(d1)/λ1| by using approximatecoefficient C₀ which makes Pearson's correlation function R satisfy0.99998≦R after these approximations. $\begin{matrix}{{di} = {\sum\limits_{k = 0}^{5}{\left( {C_{2k} \cdot h_{i}^{2k}} \right)\left( {{i = 1},2,{3\ldots\quad m}} \right)}}} & (1) \\{{di} = {\sum\limits_{k = 0}^{m - 3}{\left( {C_{2k} \cdot h_{i}^{2k}} \right)\left( {{i = 1},2,{3\ldots\quad m}} \right)}}} & (2)\end{matrix}$

In the aforesaid expression, C_(2k) represents a constant, k representsan integer, i represents a natural number and m represents the number ofthe step differences each being between paired ring-shaped zones.

As stated above, a depth parallel to the optical axis betweenring-shaped zones is determined by the diffraction efficiencies forlight fluxes with wavelengths λ1 to λ3. When blaze wavelength λBrepresents a wavelength of light that makes the diffraction efficiencyto be 100%, and mB represents its diffraction order, the light flux thathas passed through neighboring ring-shaped zones has an optical pathlength difference of λB×mB. The optical path length difference is adifference of a distance of light parallel to the direction of light,and an amount of step difference (=d, see FIG. 4) at the position thatis away from each of the optical axis where the entering light flux andthe emerging light flux for diffractive structure DOE forms an anglewith the optical axis is shifted from mB×λB/(n_(A)′−n_(B)′) (=1 see FIG.5) (where λB is blaze wavelength, n_(A)′ and n_(B)′ are refractiveindexes of medium A and medium B for λB, mB is diffraction order ofblaze wavelength). That amount of shifting is determined by an opticalpath difference function, when the diffractive structure is designed bythe optical path difference function. Since an optical path differencefunction for correcting spherical aberration is usually expressed by thepower series, the relation between height hi and depth di can also beexpressed by the power series. Therefore, if the relation between di andhi is expressed by approximation of the aforesaid expression (1), C₀that is a term of a constant results in representing an amount of thestep difference in a imaginary occasion that the step differences arelocated at the position whose height is zero.

Incidentally, when a tip portion of a serration of the diffractivestructure is rounded without being formed to be sharp in terms of anglethrough manufacturing errors, a point where an extended line of theoptical surface and an extended line of the step intersect each otherserves as a measure point of the aforesaid depth di.

Further, the aforesaid C₀ can also be expressed asC₀=mB×λB/(n_(A)′−n_(B)′) (expression (2)), because an angle formedbetween the optical axis and an entering light flux or an emerging lightflux for the diffractive structure DOE is zero, at the position where aheight from the optical axis is zero.

In this case, when n_(A) represents the refractive index of medium A forthe light flux with wavelength λ1 [mm], n_(B) represents the refractiveindex of medium B for the light flux with wavelength λ1 [mm],α=C₀×n_(d1)/λ1 can be modified to C₀=α×λ1/n_(d1)=α×λ1/(n_(A)−n_(B))(expression (3)), and when expression (2) is compared with expression(3), mB≠α, namely, that α is shifted from mB (diffraction order(integer) of blaze wavelength) can be understood, from the relation ofλB ≠λ1 and n_(A)′−n_(B)′≠n_(A)−n_(B).

Further, change of refractive index difference nd between medium A onthe light entering side and medium B on the light emerging side(n_(d2)−n_(d1))/(λ2−λ1) can be modified to{(n_(A)′−n_(B)′)−(n_(A)−n_(B))}/(λB−λ1), and57≦|(n_(d2)−n_(d1))/(λ2−λ1)|≦90 is satisfied. Therefore, causing α to bewithin the aforesaid range is to limit the range of λB as a result.

Further, a shifting amount of α from mB becomes smaller when mB growsgreater, because the decline of diffraction efficiency for changes inwavelength is greater for the higher order diffraction and it requiresto enhance diffraction efficiency for light with wavelength λ1 under thecondition of working wavelength λ1≈λB.

Since α is a numerical value close to mλ, an integer part of α is mλ.

If an optical path difference is exactly an integer multiple ofwavelength λ1, the relation of α=an integer holds, and a decline of anamount of light that is caused when the first light flux passes throughobjective lens OBJ is controlled and an amount of light in substantially100% can be secured, while in the case of α≠an integer, a decline of anamount of light corresponding to a value of α is caused.

Specifically, if ranges of 3.38≦α≦3.45, 5.37≦α≦5.46, 7.25≦α≦7.39,9.38≦α≦9.45 and 11.41≦α≦11.43 are kept, sufficient amount of light canbe secured for reproducing only of information for HD, although anamount of the first light flux is declined, and sufficient amount oflight of the second and third light fluxes for reproducing and/orrecording of information for DVD and CD can be secured.

Further, in diffractive structure DOE, distance d of the step differenceis established so that an integer part of a may become an odd number(preferably 3), in other words, the step difference may give nearly oddmultiples of wavelength λ1 to the first light flux. Owing to this,diffraction efficiency of diffracted light (for example, +third orderdiffracted light) whose diffraction order is an odd number forwavelength 407 nm (where refractive index for wavelength 407 nm of anobjective lens on which the diffractive structure DOE is formed is1.559806) becomes 100% substantially, and when the second light flux(where refractive index for wavelength 655 nm of an objective lens onwhich the diffractive structure DOE is formed is 1.540725) enters thediffractive structure DOE, +second order diffracted light with thediffraction efficiency of 88% is generated, whereby, sufficientdiffraction efficiency can be obtained in all wavelength zones for thefirst and second light fluxes.

On the other hand, if the third light flux (where refractive index forwavelength 785 nm of an objective lens on which the diffractivestructure DOE is formed is 1.537237) enters the diffractive structureDOE, +second order diffracted light and +third order diffracted lightare generated under substantially the same diffraction efficiency whichis about 40%. In this case, when an example that the second orderdiffracted light having a smaller amount of spherical aberration at themagnification identical to the optical system magnification m1 amongthese second order diffracted light and third order diffracted light isused for reproducing and/or recording for CD, is compared with anexample that distance d of the step difference is set so as to give anoptical path difference in nearly even multiples of wavelength λ1 to thefirst light flux, and a diffracted light having the maximum diffractionefficiency among third light fluxes generated in the course of passingthrough this diffractive structure, is used for reproducing and/orrecording for CD, an optical system magnification for correctingspherical aberration is closer to zero in the former, and aberrationcaused in the course of tracking can be made smaller.

Further, the diffractive structure DOE is expressed by an optical pathdifference to be given to a transmission wave front by this structure,and this optical path difference is expressed by optical path differencefunction φ(h) (mm) defined by substituting prescribed coefficients inthe following expression (Numeral 1), under the assumption that h (mm)represents a height in the direction perpendicular to the optical axis,B_(2i) represents an optical path difference function coefficient and irepresents a natural number. $\begin{matrix}{{\phi(h)} = {\sum\limits_{i = 0}{B_{2i}h^{2i}}}} & \left\lbrack {{Numeral}\quad 1} \right\rbrack\end{matrix}$

In the present embodiment, B₄ in the expression of Numeral 1 isestablished to satisfy B₄<0, so that the diffractive structure DOE mayhave positive diffractive actions.

Meanwhile, it is preferable to satisfy −1.0×10⁻³<B₄<−1.0×10⁻⁴, and it ismore preferable to satisfy −7.0×10⁻⁴<B₄<−4.5×10⁻⁴.

By establishing the diffractive structure DOE as stated above, it ispossible to give positive diffractive actions to at least one light flux(the first light flux having wavelength λ1 in the present embodiment)among light fluxes having respectively wavelength λ1, wavelength λ2 andwavelength λ3 which pass through the diffractive structure, and tocontrol an amount of changes of wavefront aberration caused bywavelength fluctuations of the first light flux resulting from ambienttemperature changes, whereby, an objective lens excellent in temperaturecharacteristics can be obtained.

Specifically, it is established so as to satisfy ΔW≦0.05 where ΔW [λrms]is a wavefront aberration change amount generated when a wavelength ofthe light flux with wavelength λ1 is fluctuated by +5 nm by ambienttemperature changes.

Further, it is preferable to cause the diffractive structure to havenegative diffraction power, and due to this, chromatic aberration of thelight flux with wavelength λ1 or with wavelength λ2 in the case ofconducting reproducing and/or recording of information for HD and DVDcan be corrected.

To be concrete, it is possible to correct chromatic aberration of thelight flux having wavelength λ1, and to obtain objective lens OBJexcellent in wavelength characteristics, by setting diffractivestructure DOE so as to satisfy |dfb/dλ|≦0.1 [μm/nm] where amount ofchange dfb/dλ at the position where the wavefront aberration in theoptical axis direction per wavelength change of 1 nm for the light fluxwith wavelength λ1 is minimum. It is further possible to correctchromatic aberration of the light flux having wavelength λ2, and toobtain objective lens OBJ excellent in wavelength characteristics, bysetting diffractive structure DOE so as to satisfy |dfb/dλ|≦0.1 [μm/nm]where amount of change dfb/dλ at the position where the wavefrontaberration in the optical axis direction per wavelength change of 1 nmfor the light flux with wavelength λ2 is minimum.

In the present embodiment, first light source LD1 and third light sourceLD3 are arranged separately each other and are arranged on optical axisL, whereby, sine conditions of objective lens OBJ are satisfied for ahigh density optical disc having a narrow tolerance.

Therefore, when a high density optical disc is used, even when gentleconverged light, for example, enters objective lens OBJ, coma caused bytracking of objective lens OBJ is not problematic. Further magnificationis smaller among magnification and sine conditions both causing coma inthe case of tracking of objective lens OBJ, although sine conditions arenot satisfied for CD because a protective layer thickness and an opticalsystem magnification of the objective lens are different greatly eachother for a high density optical disc. Therefore, coma becomes on thelevel to be used sufficiently for reproduction and/or recording, and anobjective lens excellent in tracking characteristics can be obtained.

Incidentally, when coma in the case of tracking further needs to becorrected, a coma correcting element may be provided on the light sourceside of objective lens OBJ, or, a collimator lens or a coupling lenshaving a correcting function may be provided.

It is further possible to make an arrangement wherein aperturerestricting element AP is arranged in the vicinity of optical surface S1of objective lens OBJ as an aperture element for conducting aperturerestriction corresponding to NA3, and the aperture restricting elementAP and objective lens OBJ are driven solidly for tracking by a biaxialactuator.

On the optical surface of the aperture restricting element AP in thiscase, there is formed wavelength selection filter WF havingwavelength-selectivity of transmittance. Since this wavelength selectionfilter WF has the wavelength-selectivity of transmittance wherein allwavelengths from the first wavelength λ1 to the third wavelength λ3 aretransmitted in the area within NA3, and the third wavelength λ3 only isintercepted and the first wavelength λ1 and the second wavelength λ2 aretransmitted in the area from NA3 to NA1, it is possible to conductaperture restriction corresponding to NA3 with thiswavelength-selectivity.

As a method of restriction an aperture, it is possible to employ notonly a method to use wavelength selection filter WF but also a method toswitch an aperture mechanically and a method to use liquid crystal phasecontrol element LCD.

Though it is preferable, from the viewpoint of light weight and lowprice, that objective lens OBJ is made of plastic, it may also be madeof glass from the viewpoint of moisture resistance and light stability.What is available on the market now is a refraction-type glass moldaspheric lens, but, if low-melting glass which is under development isused, a glass mold lens on which a diffractive structure is provided mayalso be manufactured. Further, in circumstances where optical plasticsare under development, there is provided a material whose refractiveindex change caused by temperature variation is small. This material isone to reduce a temperature-caused change of refractive index of totalresins by mixing inorganic microparticles having an opposite sign fortemperature-caused refractive index change therein, and there is anothermaterial wherein dispersion of total resins is made small by mixinginorganic microparticles causing small dispersion, thus, it is moreeffective if the aforesaid materials are used for an objective lens forBD.

By forming a diffractive structure whose cross-section is serrated on anoptical surface of an objective lens, and by setting it so that, forexample, 5.37≦α≦5.46 may be satisfied, as shown in the presentembodiment, it is possible to obtain an objective lens and an opticalpickup apparatus which make it possible to conduct exclusivelyreproducing of information for the first optical disc and to conductproperly reproducing and/or recording of information for the second andthird optical discs.

EXAMPLES

Next, an example of the objective lens shown in the aforesaid embodiment(Example 1) will be explained.

Lens data of the Example 1 are shown in Table 1. TABLE 1 Example 1 Lensdata Focal length of objective lens f₁ = 3.2 mm f₂ = 3.29 mm f₃ = 3.27mm Numerical aperture on the image surface side NA1: 0.65 NA2: 0.65 NA3:0.51 Diffraction order of second surface n1: 3 n2: 2 n3: 2 Magnificationm1: 0 m2: 1/87.7 m3: 0 di ni di ni di ni i^(th) surface ri (407 nm) (407nm) (655 nm) (655 nm) (785 nm) (785 nm) 0 ∞ −285.98 ∞ 1(aperture ∞ 0.10.1 0.1 diameter) (φ4.16 mm) (φ4.23 mm) (φ3.33 mm) 2   2.02281 1.900001.524392 1.90000 1.506498 1.90000 1.504969 3 −9.30563 1.73 1.0 1.76 1.01.39 1.0 4 ∞ 0.6 1.61869 0.6 1.57752 1.2 1.57063 5 ∞ *The symbol diexpresses a displacement from i^(th) surface to (i + 1)^(th) surfaceAspheric surface data Second surface Aspheric surface coefficient κ−4.3775 × E−1 A4 −1.7082 × E−3 A6 −1.3798 × E−3 A8 +5.7377 × E−4 A10−1.4792 × E−4 A12 +1.9844 × E−5 A14 −2.2752 × E−6 Optical pathdifference function (blaze wavelength 461 nm) A2 −3.0692 × E−3 A4−4.4781 × E−4 A6 −3.7423 × E−5 A8 −2.9211 × E−6 A10 +1.1336 × E−7 Thirdsurface Aspheric surface coefficient κ −1.9188 × E+2 A4 −1.1883 × E−2 A6+1.3554 × E−2 A8 −5.9377 × E−3 A10 +1.3343 × E−3 A12 −1.6015 × E−4 A14+8.0822 × E−6

As shown in Table 1, an objective lens of the example is an objectivelens of a single lens type for HD/DVD/CD compatibility, and its focallength f1 is set to 3.20 mm, magnification m1 is set to 0 and NA1 is setto 0.65 when wavelength λ1 is 407 nm, its focal length f2 is set to 3.29mm, magnification m2 is set to 1/87.7 and NA2 is set to 0.65 whenwavelength λ2 is 655 nm, and its focal length f3 is set to 3.27 mm,magnification m3 is set to 0 and NA3 is set to 0.51 when wavelength λ3is 785 nm.

The objective lens in the example is one manufactured by using“ZEONEX340R” (product name) made by ZEON Corporation as an opticalmaterial.

Each of the optical surface (second surface) on the light source sideand the optical surface (third surface) on the optical disc side, bothof the objective lens is formed to be an aspheric surface that isprescribed by the numerical expression wherein coefficients shown inTable 1 are substituted in the following Numeral 2, and is axis symmetryabout optical axis L. $\begin{matrix}{x = {\frac{h^{2}/r}{1 + \sqrt{1 - {\left( {1 + \kappa} \right)\left( {h/r^{2}} \right)}}} + {\sum\limits_{i = 2}{A_{2i}h^{2i}}}}} & \left\lbrack {{Numeral}\quad 2} \right\rbrack\end{matrix}$

In the aforesaid expression, x represents an axis (sign of lighttraveling direction is positive) in the optical axis direction, κrepresents a conic constant and A_(2i) represents an aspheric surfacecoefficient.

On each of the second and third surfaces, there is formed diffractivestructure DOE. This diffractive structure DOE is expressed by an opticalpath difference that is given to a transmitting wavefront by theaforesaid structure. This optical path difference is expressed byoptical path difference function φ(h) (mm) defined by substitutingcoefficients shown in Table 1 in the expression of the following numeral3, under the assumption that h (mm) represents a height in the directionperpendicular to the optical axis, B_(2i) represents an optical pathdifference function coefficient, n represents a diffraction order numberof diffracted light having the maximum diffraction efficiency amongdiffracted lights of incident light flux, λ (nm) represents a wavelengthof a light flux entering the diffractive structure, λB (nm) represents amanufacturing wavelength of the diffractive structure and λB representsblaze wavelength (1.0 mm in this example) of diffractive structure DOE.

[Numeral 3]

Optical Path Difference Function${\phi(h)} = {\left( {\sum\limits_{i = 0}^{5}{A_{2i}h^{2i}}} \right) \times n \times {\lambda/\lambda}\quad B}$

Incidentally, substituting (A_(2i)×n×λ/λB) represented in Numeral 3 byB_(2i) provides aforesaid Numeral 1. Namely, the relation ofA_(2i)×n×λ/λ=B_(2i) holds.

Table 2 shows heights from the optical axis (vertical distance) of stepdifferences between every pair of the ring-shaped zones in a pluralityof ring-shaped zones, depths di of the step differences parallel to theoptical axis, C₀ through C₁₀, values of approximate expression andvalues of R and α.

Values of C₀ through C₁₀ are those obtained by approximating with aleast approximation method by the use of expression (1), concerningheights from the optical axis (vertical distance) of step differencesbetween every pair of the ring-shaped zones and depths of stepdifferences parallel to the optical axis. Further, values of theapproximate expression are values of depths parallel to the optical axisat the vertical distance hi from the optical axis that are calculated byusing C_(2k) obtained by expression (1) and approximation. R representsa value of Pearson's correlation coefficient in the case where a stepdifferences having the maximum and minimum differences between a valueof the approximate expression and a value of di are excluded and allother step differences are approximated again by using expression (1).TABLE 2 Step Height from Depth of step Value of difference optical axisdifference approximate No. [mm] [mm] expression [mm]  1 0.3834140.002690 0.002690  2 0.536654 0.002709 0.002709  3 0.650749 0.0027280.002728  4 0.744228 0.002746 0.002746  5 0.824369 0.002764 0.002764  60.894957 0.002782 0.002782  7 0.958263 0.002800 0.002800  8 1.015790.002818 0.002818  9 1.06858 0.002836 0.002836 10 1.1174 0.0028530.002853 11 1.16286 0.002871 0.002871 12 1.20539 0.002888 0.002888 131.24537 0.002906 0.002906 14 1.2831 0.002923 0.002923 15 1.318830.002941 0.002941 16 1.35276 0.002958 0.002958 17 1.38506 0.0029760.002976 18 1.4159 0.002993 0.002993 19 1.44539 0.003010 0.003010 201.47365 0.003028 0.003028 21 1.50078 0.003045 0.003045 22 1.526870.003062 0.003062 23 1.55199 0.003080 0.003080 24 1.57621 0.0030970.003097 25 1.5996 0.003114 0.003114 26 1.62221 0.003131 0.003131 271.6441 0.003148 0.003148 28 1.66529 0.003165 0.003165 Coefficients ofapproximate expression (R = 1.0000) C₀ 2.6710E−03 C₂ 1.2772E−04 C₄1.4265E−05 C₆ −1.8533E−06 C₈ 2.1583E−06 C₁₀ −3.5171E−07α = C₀ × n_(d1)/λ1 = 0.002671 × (1.524392 − 1)/0.000407 = 3.441 (nd2 −nd1)/(λ2 − λ1) = {(1.506498 − 1) − (1.524392 − 1)}/(0.000655 − 0.000407)= −72.153

As shown in Table 2, the objective lens of this example is establishedso that a value of α calculated by using coefficient C₀ of theapproximate expression in the case of R=1.0000 is 3.441, and itsatisfies a relational expression of 3.38≦β≦3.45.

Next, an example (Example 2) of the objective lens shown in theaforesaid example will be explained.

Table 3 shows lens data of Example 2. TABLE 3 Example 2 Lens data Focallength of objective lens f₁ = 3.0 mm f₂ = 3.10 mm f₃ = 3.16 mm Numericalaperture on the image surface side NA1: 0.65 NA2: 0.65 NA3: 0.51Diffraction order of second surface n1: 5 n2: 3 n3: 2 Magnification m1:1/50.8 m2: 1/49.3 m3: −1/32.4 di ni di ni di ni i^(th) surface ri (407nm) (407 nm) (655 nm) (655 nm) (785 nm) (785 nm) 0 −150.00 −150.00105.00 1(aperture ∞ 0.1 0.1 0.1 diameter) (φ3.84 mm) (φ3.97 mm) (φ3.31mm) 2 6.49242 1.00000 1.559806 1.00000 1.540725 1.00000 1.537237 311.40378 0.05 1.0 0.05 1.0 0.05 1.0 4 1.85946 1.80000 1.559806 1.800001.540725 1.80000 1.537237 5 29.37511 1.16 1.0 1.23 1.0 0.97 1.0 6 ∞ 0.61.61869 0.6 1.57752 1.2 1.57063 7 ∞ *The symbol di expresses adisplacement from i^(th) surface to (i + 1)^(th) surface Asphericsurface data Second surface Aspheric surface coefficient κ −2.6300 × E+1A4 +1.5183 × E−3 A6 +1.0648 × E−4 A8 +5.1422 × E−5 A10 −8.1669 × E−6Optical path difference function (blaze wavelength 435 nm) A2 −2.5670 ×E−3 A4 −1.3664 × E−4 A6 +2.5090 × E−5 A8 −5.3822 × E−6 A10 +2.7125 × E−7Third surface Aspheric surface coefficient κ −1.0000 × E+2 A4 −2.9409 ×E−3 A6 +1.2467 × E−3 A8 −7.4787 × E−5 A10 −2.0947 × E−6 Fourth surfaceAspheric surface coefficient κ −3.4445 × E−1 A4 +4.3745 × E−3 A6 −7.0603× E−4 A8 +4.8641 × E−4 A10 −5.9636 × E−5 A12 +1.1486 × E−5 A14 −1.8478 ×E−6 Fifth surface Aspheric surface coefficient κ −2.7392 × E+2 A4+2.6157 × E−2 A6 −2.3051 × E−2 A8 +2.1017 × E−2 A10 −1.3047 × E−2 A12+4.3440 × E−3 A14 −5.9266 × E−4

As shown in Table 3, an objective lens of this is an objective lens of atwo-group two-element type for HD/DVD/CD compatibility, and its focallength f1 is set to 3.00 mm, magnification m1 is set to 1/50.8 and NA1is set to 0.65 when wavelength λ1 is 407 nm, its focal length f2 is setto 3.10 mm, magnification m2 is set to 1/49.3 and NA2 is set to 0.65when wavelength λ2 is 655 nm, and its focal length f3 is set to 3.16 mm,magnification m3 is set to − 1/32.4 and NA3 is set to 0.51 whenwavelength λ3 is 785 nm.

The objective lens in the present embodiment is one manufactured byusing “APEL” (product name) made by Mitsui Petrochemical Industries,Ltd. as an optical material.

In two lenses forming the objective lens, each of the optical surface(second surface) arranged on the light source side and the opticalsurface (third surface) arranged on the optical disc side both in thelens arranged at the light source side and the optical surface (fourthsurface) arranged on the light source side and the optical surface(fifth surface) arranged on the optical disc side both in the lensarranged at the optical disc side, is formed to be an aspheric surfacethat is prescribed by the numerical expression wherein coefficientsshown in Table 3 are substituted for the aforesaid Numeral 2, and isaxis symmetry about optical axis L.

On the second surface, there is formed diffractive structure DOE. Thisdiffractive structure DOE is expressed by an optical path differencethat is given to a transmitting wavefront by this structure. Thisoptical path difference is expressed by optical path difference functionφ(h) (mm) defined by substituting coefficients shown in Table 3 for theaforesaid Numeral 3 expression.

Table 4 shows heights from the optical axis (vertical distance) of stepdifferences between every pair of the ring-shaped zones in a pluralityof ring-shaped zones, depths di of the step differences parallel to theoptical axis, C₀ through C₁₀, values of approximate expression andvalues of R and α.

Values of C₀ through C₁₀ are those obtained by approximating with aleast approximation method by the use of expression (1), concerningheights from the optical axis (vertical distance) of step differencesbetween every pair of the ring-shaped zones and depths of stepdifferences parallel to the optical axis. Further, values of theapproximate expression are values of depths parallel to the optical axisat the vertical distance hi from the optical axis that are calculated byusing C_(2k) obtained by expression (1) and approximation. R representsa value of Pearson's correlation coefficient in the case where stepdifferences having the maximum and minimum differences between a valueof the approximate expression and a value of di are excluded and allother step differences are approximated again by using expression (1).TABLE 4 Value of Step Height from Depth of step approximate differenceoptical axis difference expression No. [mm] [mm] [mm]  1 0.4098780.003918 0.003918  2 0.577354 0.003923 0.003923  3 0.704508 0.0039270.003927  4 0.8107 0.003931 0.003931  5 0.903464 0.003935 0.003935  60.986674 0.003938 0.003938  7 1.06263 0.003942 0.003942  8 1.132840.003945 0.003945  9 1.19834 0.003948 0.003948 10 1.25989 0.0039520.003952 11 1.31803 0.003955 0.003955 12 1.37323 0.003959 0.003959 131.42581 0.003962 0.003962 14 1.47606 0.003966 0.003966 15 1.524210.003969 0.003969 16 1.57043 0.003973 0.003973 17 1.6149 0.0039770.003977 18 1.65774 0.003981 0.003981 Coefficients of approximateexpression (R = 1.0000) C₀ 3.9123E−03 C₂ 3.7087E−05 C₄ −1.9130E−05 C₆1.1579E−05 C₈ −3.4096E−06 C₁₀ 4.1753E−07 α = C₀ × n_(d1)/λ1 = 0.0039123× (1.559806 − 1)/0.000407 = 5.381 (nd2 − nd1)/(λ2 − λ1) = {(1.540725− 1) − (1.559806 − 1)}/ (0.000655 − 0.000407) = −76.940

As shown in Table 4, the objective lens of this example is establishedso that a value of α calculated by using coefficient C₀ of theapproximate expression in the case of R=1.0000 is 5.381, and itsatisfies a relational expression of 5.37≦α≦5.46.

The expression of Pearson's correlation coefficient (least squaresmethod) is shown in Numeral 4. $\begin{matrix}{R = \frac{\sum\limits_{i}{\left( {h_{i} - \overset{\_}{h}} \right)\left( {d_{i} - \overset{\_}{d}} \right)}}{\sqrt{\sum\limits_{i}\left( {h_{i} - \overset{\_}{h}} \right)^{2}}\sqrt{\left. {{\sum\limits_{i}d_{i}} - \overset{\_}{d}} \right)^{2}}}} & \left\lbrack {{Numeral}\quad 4} \right\rbrack\end{matrix}$

INDUSTRIAL APPLICABILITY

The present invention provides an objective lens for use in an opticalpickup apparatus exclusively reproducing information for a high densityoptical disc and reproducing and/or recording information for the othertwo types of optical discs, with excellent wavelength characteristicsand tracking characteristics, and an optical pickup apparatus employingthis objective lens.

1. An objective lens for an optical pickup apparatus at leastreproducing information using a light flux with a wavelength λ1 emittedfrom a first light source for a first optical disc having a protectivesubstrate with a thickness t1, recording and/or reproducing informationusing a light flux with a wavelength λ2 (1.5×λ1≦λ2≦1.7×λ1) emitted froma second light source for a second optical disc having a protectivesubstrate with a thickness t2, and recording and/or reproducinginformation using a light flux with a wavelength λ3 (1.8×λ1≦λ3≦2.2×λ1)emitted from a third light source for a third optical disc having aprotective substrate with a thickness t3 (1.9×t1≦t3≦2.1×t1), theobjective lens comprising: at least one optical surface including acommon area for reproducing information for the first disc and forrecording and/or reproducing information for the second and third discs;and a diffractive structure arranged on the common area, having a crosssection in a serrated shape and divided into a plurality of ring-shapedzones whose centers are on an optical axis, wherein when the opticalsurface including the diffractive structure forms a border, a refractiveindex difference n_(d1) for a light flux with the wavelength λ1 betweena medium A arranged at a light source side of the optical surface and amedium B arranged at an optical disc side of the optical surface, andrefractive index difference n_(d2) for a light flux with the wavelengthλ2 between the medium A and the medium B satisfies57≦|(n _(d2) −n _(d1))/(λ2−λ1)|≦90, and wherein when the plurality ofring-shaped zones have step differences between every pair of thering-shaped zones each having a depth di [mm] parallel to an opticalaxis and a vertical distance hi [mm] from the optical axis, theobjective lens satisfies any one of:3.38≦α≦3.45,5.37≦α≦5.46,7.25≦α≦7.39,9.38≦α≦9.45, and11.41≦α≦11.43, where α is defined byα==C ₀ ×|n _(d1)/λ1| using an approximate coefficient C₀ which makes aPearson's correlation coefficient R satisfy 0.99998≦R when, for the stepdifferences whose number m satisfies m>7 arranged between every pair ofthe ring-shaped zones, a depth d parallel to the optical axis at avertical distance hi from the optical axis is approximated by a leastsquares method using a following expression (1) to obtain C_(2k) (k isan integer in a range of 0 to 5) and is determined by using C_(2k), anda whole of the step differences except step differences providingmaximum and minimum differences between a calculated value of the depthd parallel to the optical axis at the vertical distance hi from theoptical axis and di, are approximated again using the expression (1),wherein for the step differences whose number m satisfies m≦7 arrangedbetween every pair of the ring-shaped zones, when the depth d parallelto the optical axis at a vertical distance hi from the optical axis isdetermined, C_(2k) (k is an integer in a range of 0 to m−3) is obtainedby approximating by the least squares method using a followingexpression (2) instead of the expression (1), and wherein when a numberof the calculated values of the depth d parallel to the optical axis ata vertical distance hi from the optical axis is 3 or less, a whole ofthe step differences including the step differences providing themaximum and minimum differences between the calculated value of thedepth d parallel to the optical axis at a vertical distance hi from theoptical axis and di, are further approximated, where $\begin{matrix}{{{di} = {\sum\limits_{k = 0}^{5}{\left( {C_{2k} \cdot h_{i}^{2k}} \right)\left( {{i = 1},2,{3\ldots\quad m}} \right)}}},} & (1) \\{{{di} = {\sum\limits_{k = 0}^{m - 3}{\left( {C_{2k} \cdot h_{i}^{2k}} \right)\left( {{i = 1},2,{3\ldots\quad m}} \right)}}},} & (2)\end{matrix}$ C_(2k) is a constant, k is an integer, i is a naturalnumber, and m is a number of the step differences between every pair ofthe ring-shaped zones.
 2. The objective lens of claim 1, wherein α hasan integer part of
 3. 3. The objective lens of claim 1, wherein theobjective lens is formed of two lenses.
 4. The objective lens of claim3, wherein the diffractive structure is on a lens arranged at a lightsource side between the two lenses.
 5. The objective lens of claim 1,wherein the objective lens satisfies60≦|(n _(d2) −n _(d1))/(λ2−λ1)|≦80.
 6. The objective lens of claim 1,wherein the diffractive structure is defined by using an optical pathdifference function φ(h) whereφ(h)=B ₂ ×h ² +B ₄ ×h ⁴ + . . . +B _(2i) ×h ^(2i), andB₄<0 where B_(2i) is a coefficient of the optical path differencefunction and i is a natural number.
 7. The objective lens of claim 1,wherein the diffractive structure has a negative diffractive power. 8.The objective lens of claim 7, wherein a change amount of a positiondfb/dλ where a wavefront aberration is minimum along the optical axis inthe objective lens for a light flux with the wavelength λ1 per awavelength change of 1 nm satisfies|dfb/dλ|≦0.1 [μm/nm], where fb is a distance between the objective lensand the first optical disc.
 9. The objective lens of claim 7, wherein achange amount of a position dfb/dλ where a wavefront aberration isminimum along the optical axis in the objective lens for a light fluxwith the wavelength λ2 per a wavelength change of 1 nm satisfies|dfb/dλ|≦0.1 [μm/nm], where fb is a distance between the objective lensand the second optical disc.
 10. The objective lens of claim 1, wherein,among optical system magnifications m1, m2, and m3 of the objective lensfor the wavelength λ1, λ2 and λ3, m1=0 and at least one of m2 and m3satisfies a following expression:1/100<|m|≦ 1/10.
 11. The objective lens of claim 1, wherein t1=t2. 12.The objective lens of claim 1, wherein a focal length f1 of theobjective lens for a light flux with the wavelength λ1 satisfies0.8 mm≦f1≦4.0 mm.
 13. The objective lens of claim 1, wherein theobjective lens is formed of a plastic.
 14. The objective lens of claim1, wherein the objective lens satisfies 0.9×t1≦t2≦1.1×t1.
 15. Theobjective lens of claim 1, wherein one of the medium A and medium B isan air and the other is a lens material of the lens including thediffractive structure.
 16. An objective lens for an optical pickupapparatus, at least reproducing information using a light flux with awavelength λ1 emitted from a first light source for a first optical dischaving a protective substrate with a thickness t1, recording and/orreproducing information using a light flux with a wavelength λ2(1.5×λ1≦λ2≦1.7×λ1) emitted from a second light source for a secondoptical disc having a protective substrate with a thickness t2, andrecording and/or reproducing information using a light flux with awavelength λ3 (1.8×λ1≦λ3≦2.2×λ1) emitted from a third light source for athird optical disc having a protective substrate with a thickness t3(1.9×t1≦t3≦2.1×t1), the objective lens comprising: at least one opticalsurface having a common area for reproducing information for the firstdisc and for recording and/or reproducing information for the second andthird discs; and a diffractive structure arranged on the common areawhose diffraction efficient of the light flux with a wavelength λ1 forthe first optical disc is 50% or more, and whose diffraction efficientof a light flux with the wavelength λ2 for the second optical disc is70% or more.
 17. The objective lens of claim 16, wherein the diffractivestructure has a cross section in a serrated shape is divided into aplurality of ring-shaped zones whose centers are on an optical axis. 18.The objective lens of claim 16, wherein when the optical surfaceincluding the diffractive structure forms a border, a refractive indexdifference n_(d1) for a light flux with the wavelength λ1 between amedium A arranged at a light source side of the optical surface and amedium B arranged at an optical disc side of the optical surface, andrefractive index difference n_(d2) for a light flux with the wavelengthλ2 between the medium A and the medium B satisfy57≦|(n _(d2) −n _(d1))/(λ2−λ1)|≦90.
 19. The objective lens of claim 17,wherein when the plurality of ring-shaped zones have step differencesbetween every pair of the ring-shaped zones each having a depth di [mm]parallel to an optical axis and a vertical distance hi [mm] from theoptical axis, the objective lens satisfies any one of:3.38≦α≦3.45,5.37≦α≦5.46,7.25≦α≦7.39,9.38≦α≦9.45, and11.41≦α≦11.43, where α is defined byα==C ₀ ×|n _(d1)/λ1| using an approximate coefficient C₀ which makes aPearson's correlation coefficient R satisfy 0.99998≦R when for the stepdifferences whose number m satisfies m>7 arranged between every pair ofthe ring-shaped zones, a depth d parallel to the optical axis at avertical distance hi from the optical axis is approximated by a leastsquares method using a following expression (1) to obtain C_(2k) (k isan integer in a range of 0 to 5) and is determined by using C_(2k), anda whole of the step differences except step differences providingmaximum and minimum differences between a calculated value of the depthd parallel to the optical axis at the vertical distance hi from theoptical axis and di, are further approximated using the expression (1),wherein for the step differences whose number m satisfies m≦7 arrangedbetween every pair of the ring-shaped zones, when the depth d parallelto the optical axis at a vertical distance hi from the optical axis isdetermined, C_(2k) (k is an integer in a range of 0 to m−3) is obtainedby approximating by the least squares method using a followingexpression (2) instead of the expression (1), and wherein when a numberof the calculated values of the depth d parallel to the optical axis ata vertical distance hi from the optical axis is 3 or less, a whole ofthe step differences including the step differences providing themaximum and minimum differences between the calculated value of thedepth d parallel to the optical axis at a vertical distance hi from theoptical axis and di, are approximated again, where $\begin{matrix}{{{di} = {\sum\limits_{k = 0}^{5}{\left( {C_{2k} \cdot h_{i}^{2k}} \right)\left( {{i = 1},2,{3\ldots\quad m}} \right)}}},} & (1) \\{{{di} = {\sum\limits_{k = 0}^{m - 3}{\left( {C_{2k} \cdot h_{i}^{2k}} \right)\left( {{i = 1},2,{3\ldots\quad m}} \right)}}},} & (2)\end{matrix}$ C_(2k) is a constant, k is an integer, i is a naturalnumber, and m is a number of the step differences between every pair ofthe ring-shaped zones.
 20. The objective lens of claim 19, wherein α hasan integer part of
 3. 21. The objective lens of claim 16, wherein theobjective lens is formed of two lenses.
 22. The objective lens of claim21, wherein the diffractive structure is on a lens arranged at a lightsource side between the two lenses.
 23. The objective lens of claim 18,wherein the objective lens satisfies60≦|(n _(d2) −n _(d1))/(λ2−λ1)|≦80.
 24. An optical pickup apparatuscomprising: a first light source for reproducing information using alight flux with a wavelength λ1 emitted from the first light source fora first optical disc having a protective substrate with a thickness t1;a second light source for recording and/or reproducing information usinga light flux with a wavelength λ2 (1.5×λ1≦λ2≦1.7×λ1) emitted from thesecond light source for a second optical disc having a protectivesubstrate with a thickness t2; a third light source for recording and/orreproducing information using a light flux with a wavelength λ3(1.8×λ1≦λ3≦2.2×λ1) emitted from the third light source for a thirdoptical disc having a protective substrate with a thickness t3(1.9×t1≦t3≦2.1×t1); and the objective lens of claim
 1. 25. An opticalpickup apparatus comprising: a first light source for reproducinginformation using a light flux with a wavelength λ1 emitted from thefirst light source for a first optical disc having a protectivesubstrate with a thickness t1; a second light source for recordingand/or reproducing information using a light flux with a wavelength λ2(1.5×λ1≦λ2≦1.7×λ1) emitted from the second light source for a secondoptical disc having a protective substrate with a thickness t2; a thirdlight source for recording and/or reproducing information using a lightflux with a wavelength λ3 (1.8×λ1≦λ3≦2.2×λ1) emitted from the thirdlight source for a third optical disc having a protective substrate witha thickness t3 (1.9×t1≦t3≦2.1×t1); and the objective lens of claim 16.